An elevator comprises an elevator car and a counterweight attached to each other by a series of ropes. The ropes extend up the hoistway from the elevator car to the machine room of the elevator. In the machine room, the ropes wrap around a sheave attached to an elevator drive and return down the hoistway, to the counterweight.
For safety reasons, elevators are generally required to have an overspeed governor and safeties. A typical overspeed governor often includes a governor rope extending the length of the hoistway, attached to a governor sheave and tensioner. The governor rope is fixed to the elevator car by a linkage extending from the rope to a pair of safeties attached to the car. Because of the fixed relationship, any distance traveled by the car causes the rope to travel a like amount.
In this type of arrangement, if the downward velocity of the elevator exceeds a predetermined limit, i.e., an overspeed condition, a centrifugal flyweight assembly driven by the governor sheave swings outwardly, tripping a switch thereby removing power to the elevator drive and brake. If the downward elevator speed continues to increase, the flyweight assembly swings outwardly still further and operates a governor brake. The governor brake applies a frictional drag force to the governor rope, thereby actuating the safeties attached to the elevator car. The safeties act on a pair of guiderails and the car is consequently brought to a stop safely. U.S. Pat. No. 3,327,811 to Mastroberte discloses such an arrangement.
This style overspeed governor, although effective, protects against overspeed conditions only when the elevator car is descending. In the event of a brake failure or a drive gear failure in a geared machine, for example, a heavier counterweight will cause a lighter elevator car to accelerate upwardly. The unidirectional limitation of this style governor renders it powerless to stop an upwardly accelerating car. In recent years, elevator codes have changed and now require an overspeed device capable of stopping the elevator in both the upward and downward direction. Therefore, what is needed is an overspeed device capable of stopping the elevator in both the upward and downward direction.